Thank you for reporting.
When the TuneBrowser uses SOAP to control the UPnP Renderer, it waits for a certain period of time for a response. If the next control request is received from user while waiting for its response, synchronization occurs. If the response from the device is slow, the TuneBrowser may appear to freeze as a result.
During playback with the UPnP renderer, the TuneBrowser does not perform RAMDecode because it does not directly send PCM data to the device. However, when sending files, large cache in RAM is used. Therefore, even if the UPnP renderer performs random access to the file, it is not accessing the disk each time.
I don’t know why Synology Audio Station can play DSD, but it may be sending files ignoring the supported mime type.
And just to be clear, TuneBrowser does not support DLNA; it is developed based on UPnP and OpenHome public information.